package/libpng: cleanup NEON support options

Suggested by Thomas:
http://lists.busybox.net/pipermail/buildroot/2017-April/189086.html

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/libpng/libpng.mk

@@ -14,7 +14,12 @@ LIBPNG_INSTALL_STAGING = YES
 LIBPNG_DEPENDENCIES = host-pkgconf zlib
 HOST_LIBPNG_DEPENDENCIES = host-pkgconf host-zlib
 LIBPNG_CONFIG_SCRIPTS = libpng$(LIBPNG_SERIES)-config libpng-config
-LIBPNG_CONF_OPTS = $(if $(BR2_ARM_CPU_HAS_NEON),--enable-arm-neon=yes,--enable-arm-neon=no)
+
+ifeq ($(BR2_ARM_CPU_HAS_NEON),y)
+LIBPNG_CONF_OPTS += --enable-arm-neon
+else
+LIBPNG_CONF_OPTS += --disable-arm-neon
+endif
 
 $(eval $(autotools-package))
 $(eval $(host-autotools-package))